From c297a37096ae1562d453624984aa5f924ab490a1 Mon Sep 17 00:00:00 2001
From: Jonas Platte <jplatte+git@posteo.de>
Date: Sun, 7 Sep 2025 22:44:18 +0200
Subject: [PATCH] tracing-subscriber: Switch to unconditional no_std

---
 tracing-subscriber/src/filter/directive.rs    |  7 +++---
 tracing-subscriber/src/filter/env/builder.rs  |  7 +++++-
 .../src/filter/env/directive.rs               |  2 ++
 tracing-subscriber/src/filter/env/field.rs    | 23 +++++++++++--------
 tracing-subscriber/src/filter/env/mod.rs      |  6 ++++-
 .../src/filter/layer_filters/mod.rs           |  7 +++---
 tracing-subscriber/src/filter/targets.rs      | 22 ++++++++++--------
 tracing-subscriber/src/fmt/fmt_layer.rs       | 10 ++++----
 tracing-subscriber/src/fmt/format/json.rs     |  8 ++++---
 tracing-subscriber/src/fmt/format/mod.rs      | 15 +++++++++---
 tracing-subscriber/src/fmt/mod.rs             |  8 +++++--
 .../src/fmt/time/chrono_crate.rs              |  8 +++----
 tracing-subscriber/src/fmt/time/datetime.rs   |  6 +++--
 tracing-subscriber/src/fmt/writer.rs          | 13 +++++++----
 tracing-subscriber/src/layer/mod.rs           |  3 ---
 tracing-subscriber/src/layer/tests.rs         |  2 ++
 tracing-subscriber/src/lib.rs                 |  5 +++-
 tracing-subscriber/src/registry/extensions.rs |  6 ++---
 tracing-subscriber/src/registry/mod.rs        |  5 +++-
 tracing-subscriber/src/registry/sharded.rs    |  5 +++-
 tracing-subscriber/src/registry/stack.rs      |  1 +
 tracing-subscriber/src/util.rs                |  3 +++
 22 files changed, 108 insertions(+), 64 deletions(-)

diff --git a/tracing-subscriber/src/filter/directive.rs b/tracing-subscriber/src/filter/directive.rs
index 9ee2ce5a..d2faf6ea 100644
--- a/tracing-subscriber/src/filter/directive.rs
+++ b/tracing-subscriber/src/filter/directive.rs
@@ -1,7 +1,6 @@
 use crate::filter::level::{self, LevelFilter};
-#[cfg(not(feature = "smallvec"))]
-use alloc::vec;
-#[cfg(not(feature = "std"))]
+#[cfg(feature = "std")]
+use alloc::boxed::Box;
 use alloc::{string::String, vec::Vec};
 
 use core::{cmp::Ordering, fmt, iter::FromIterator, slice, str::FromStr};
@@ -126,7 +125,7 @@ impl<T> IntoIterator for DirectiveSet<T> {
     #[cfg(feature = "smallvec")]
     type IntoIter = smallvec::IntoIter<[T; 8]>;
     #[cfg(not(feature = "smallvec"))]
-    type IntoIter = vec::IntoIter<T>;
+    type IntoIter = alloc::vec::IntoIter<T>;
 
     fn into_iter(self) -> Self::IntoIter {
         self.directives.into_iter()
diff --git a/tracing-subscriber/src/filter/env/builder.rs b/tracing-subscriber/src/filter/env/builder.rs
index 2c11edff..9cb1e401 100644
--- a/tracing-subscriber/src/filter/env/builder.rs
+++ b/tracing-subscriber/src/filter/env/builder.rs
@@ -3,7 +3,12 @@ use super::{
     EnvFilter, FromEnvError,
 };
 use crate::sync::RwLock;
-use std::env;
+use alloc::{
+    format,
+    string::{String, ToString},
+    vec::Vec,
+};
+use std::{env, eprintln};
 use thread_local::ThreadLocal;
 use tracing::level_filters::STATIC_MAX_LEVEL;
 
diff --git a/tracing-subscriber/src/filter/env/directive.rs b/tracing-subscriber/src/filter/env/directive.rs
index 7c348fd4..290b85ae 100644
--- a/tracing-subscriber/src/filter/env/directive.rs
+++ b/tracing-subscriber/src/filter/env/directive.rs
@@ -4,6 +4,7 @@ use crate::filter::{
     env::{field, FieldMap},
     level::LevelFilter,
 };
+use alloc::{borrow::ToOwned, string::String, vec::Vec};
 use std::{cmp::Ordering, fmt, iter::FromIterator, str::FromStr};
 use tracing_core::{span, Level, Metadata};
 
@@ -481,6 +482,7 @@ impl SpanMatcher {
 #[cfg(test)]
 mod test {
     use super::*;
+    use alloc::{format, string::ToString, vec};
 
     fn parse_directives(dirs: impl AsRef<str>) -> Vec<Directive> {
         dirs.as_ref()
diff --git a/tracing-subscriber/src/filter/env/field.rs b/tracing-subscriber/src/filter/env/field.rs
index 1a49b958..45348bc4 100644
--- a/tracing-subscriber/src/filter/env/field.rs
+++ b/tracing-subscriber/src/filter/env/field.rs
@@ -1,14 +1,17 @@
-use matchers::Pattern;
-use std::{
+use alloc::{
+    borrow::ToOwned,
+    boxed::Box,
+    string::{String, ToString},
+    sync::Arc,
+};
+use core::{
     cmp::Ordering,
-    error::Error,
     fmt::{self, Write},
     str::FromStr,
-    sync::{
-        atomic::{AtomicBool, Ordering::*},
-        Arc,
-    },
+    sync::atomic::{AtomicBool, Ordering::*},
 };
+use matchers::Pattern;
+use std::error::Error;
 
 use super::{FieldMap, LevelFilter};
 use tracing_core::field::{Field, Visit};
@@ -507,9 +510,7 @@ impl Visit for MatchVisitor<'_> {
             Some((ValueMatch::NaN, ref matched)) if value.is_nan() => {
                 matched.store(true, Release);
             }
-            Some((ValueMatch::F64(ref e), ref matched))
-                if (value - *e).abs() < f64::EPSILON =>
-            {
+            Some((ValueMatch::F64(ref e), ref matched)) if (value - *e).abs() < f64::EPSILON => {
                 matched.store(true, Release);
             }
             _ => {}
@@ -576,6 +577,8 @@ impl Visit for MatchVisitor<'_> {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use alloc::format;
+
     #[derive(Debug)]
     #[allow(dead_code)]
     struct MyStruct {
diff --git a/tracing-subscriber/src/filter/env/mod.rs b/tracing-subscriber/src/filter/env/mod.rs
index a9d93793..04abd591 100644
--- a/tracing-subscriber/src/filter/env/mod.rs
+++ b/tracing-subscriber/src/filter/env/mod.rs
@@ -14,8 +14,10 @@ use crate::{
     layer::{Context, Layer},
     sync::RwLock,
 };
+use alloc::{fmt, str::FromStr, vec::Vec};
+use core::cell::RefCell;
 use directive::ParseError;
-use std::{cell::RefCell, collections::HashMap, env, error::Error, fmt, str::FromStr};
+use std::{collections::HashMap, env, error::Error};
 use thread_local::ThreadLocal;
 use tracing_core::{
     callsite,
@@ -835,6 +837,8 @@ impl Error for FromEnvError {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use alloc::format;
+    use std::println;
     use tracing_core::field::FieldSet;
     use tracing_core::*;
 
diff --git a/tracing-subscriber/src/filter/layer_filters/mod.rs b/tracing-subscriber/src/filter/layer_filters/mod.rs
index 078051c7..238ec932 100644
--- a/tracing-subscriber/src/filter/layer_filters/mod.rs
+++ b/tracing-subscriber/src/filter/layer_filters/mod.rs
@@ -32,15 +32,14 @@ use crate::{
     layer::{self, Context, Layer},
     registry,
 };
-use std::{
+use alloc::{boxed::Box, fmt, sync::Arc};
+use core::{
     any::TypeId,
     cell::{Cell, RefCell},
-    fmt,
     marker::PhantomData,
     ops::Deref,
-    sync::Arc,
-    thread_local,
 };
+use std::thread_local;
 use tracing_core::{
     span,
     subscriber::{Interest, Subscriber},
diff --git a/tracing-subscriber/src/filter/targets.rs b/tracing-subscriber/src/filter/targets.rs
index 19f2d990..43198645 100644
--- a/tracing-subscriber/src/filter/targets.rs
+++ b/tracing-subscriber/src/filter/targets.rs
@@ -13,7 +13,6 @@ use crate::{
     },
     layer,
 };
-#[cfg(not(feature = "std"))]
 use alloc::string::String;
 use core::{
     fmt,
@@ -596,16 +595,17 @@ impl<'a> Iterator for Iter<'a> {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use alloc::{string::ToString, vec, vec::Vec};
+    #[cfg(feature = "std")]
+    use std::dbg;
 
-    feature! {
-        #![not(feature = "std")]
-        use alloc::{vec, vec::Vec, string::ToString};
-
-        // `dbg!` is only available with `libstd`; just nop it out when testing
-        // with alloc only.
-        macro_rules! dbg {
-            ($x:expr) => { $x }
-        }
+    // `dbg!` is only available with `libstd`; just nop it out when testing
+    // with alloc only.
+    #[cfg(not(feature = "std"))]
+    macro_rules! dbg {
+        ($x:expr) => {
+            $x
+        };
     }
 
     fn expect_parse(s: &str) -> Targets {
@@ -775,6 +775,8 @@ mod tests {
     // `println!` is only available with `libstd`.
     #[cfg(feature = "std")]
     fn size_of_filters() {
+        use std::println;
+
         fn print_sz(s: &str) {
             let filter = s.parse::<Targets>().expect("filter should parse");
             println!(
diff --git a/tracing-subscriber/src/fmt/fmt_layer.rs b/tracing-subscriber/src/fmt/fmt_layer.rs
index 9dbc0fe8..d0233810 100644
--- a/tracing-subscriber/src/fmt/fmt_layer.rs
+++ b/tracing-subscriber/src/fmt/fmt_layer.rs
@@ -4,10 +4,10 @@ use crate::{
     layer::{self, Context},
     registry::{self, LookupSpan, SpanRef},
 };
+use alloc::{fmt, format, string::String};
+use core::{any::TypeId, marker::PhantomData, ops::Deref};
 use format::{FmtSpan, TimingDisplay};
-use std::{
-    any::TypeId, cell::RefCell, env, fmt, io, marker::PhantomData, ops::Deref, time::Instant,
-};
+use std::{cell::RefCell, env, eprintln, io, thread_local, time::Instant};
 use tracing_core::{
     field,
     span::{Attributes, Current, Id, Record},
@@ -1257,6 +1257,7 @@ mod test {
         time,
     };
     use crate::Registry;
+    use alloc::{string::ToString, vec, vec::Vec};
     use format::FmtSpan;
     use regex::Regex;
     use tracing::subscriber::with_default;
@@ -1631,8 +1632,7 @@ mod test {
             .with_timer(MockTime)
             .with_span_events(FmtSpan::ACTIVE);
 
-        let (reloadable_layer, reload_handle) =
-            crate::reload::Layer::new(inner_layer);
+        let (reloadable_layer, reload_handle) = crate::reload::Layer::new(inner_layer);
         let reload = reloadable_layer.with_subscriber(Registry::default());
 
         with_default(reload, || {
diff --git a/tracing-subscriber/src/fmt/format/json.rs b/tracing-subscriber/src/fmt/format/json.rs
index 3ef0fcd6..6266b475 100644
--- a/tracing-subscriber/src/fmt/format/json.rs
+++ b/tracing-subscriber/src/fmt/format/json.rs
@@ -7,12 +7,14 @@ use crate::{
     },
     registry::LookupSpan,
 };
-use serde::ser::{SerializeMap, Serializer as _};
-use serde_json::Serializer;
-use std::{
+use alloc::{
     collections::BTreeMap,
     fmt::{self, Write},
+    format,
+    string::String,
 };
+use serde::ser::{SerializeMap, Serializer as _};
+use serde_json::Serializer;
 use tracing_core::{
     field::{self, Field},
     span::Record,
diff --git a/tracing-subscriber/src/fmt/format/mod.rs b/tracing-subscriber/src/fmt/format/mod.rs
index f72422bf..ea2a2eca 100644
--- a/tracing-subscriber/src/fmt/format/mod.rs
+++ b/tracing-subscriber/src/fmt/format/mod.rs
@@ -48,7 +48,6 @@ use tracing_log::NormalizeEvent;
 #[cfg(feature = "ansi")]
 use nu_ansi_term::{Color, Style};
 
-
 mod escape;
 use escape::Escape;
 
@@ -433,6 +432,8 @@ impl<'writer> Writer<'writer> {
     /// value implementing [`fmt::Write`] is a [`String`], it will contain
     /// the formatted output of [`Format::format_event`], which may then be
     /// used for other purposes.
+    ///
+    /// [`String`]: alloc::string::String
     #[must_use]
     pub fn new(writer: &'writer mut impl fmt::Write) -> Self {
         Self {
@@ -1269,7 +1270,10 @@ impl field::Visit for DefaultVisitor<'_> {
                 ),
             )
         } else {
-            self.record_debug(field, &format_args!("{}", Escape(&format_args!("{}", value))))
+            self.record_debug(
+                field,
+                &format_args!("{}", Escape(&format_args!("{}", value))),
+            )
         }
     }
 
@@ -1294,7 +1298,7 @@ impl field::Visit for DefaultVisitor<'_> {
             "message" => {
                 // Escape ANSI characters to prevent malicious patterns (e.g., terminal injection attacks)
                 write!(self.writer, "{:?}", Escape(value))
-            },
+            }
             name if name.starts_with("r#") => write!(
                 self.writer,
                 "{}{}{:?}",
@@ -1750,6 +1754,11 @@ impl Display for TimingDisplay {
 #[cfg(test)]
 pub(super) mod test {
     use crate::fmt::{test::MockMakeWriter, time::FormatTime};
+    use alloc::{
+        borrow::ToOwned,
+        format,
+        string::{String, ToString},
+    };
     use tracing::{
         self,
         dispatcher::{set_default, Dispatch},
diff --git a/tracing-subscriber/src/fmt/mod.rs b/tracing-subscriber/src/fmt/mod.rs
index 94ebd49c..785b6c43 100644
--- a/tracing-subscriber/src/fmt/mod.rs
+++ b/tracing-subscriber/src/fmt/mod.rs
@@ -189,7 +189,10 @@
 //!     https://docs.rs/tracing/latest/tracing/trait.Subscriber.html
 //! [`tracing`]: https://crates.io/crates/tracing
 //! [`fmt::format`]: mod@crate::fmt::format
-use std::{any::TypeId, error::Error, io};
+
+use alloc::boxed::Box;
+use core::any::TypeId;
+use std::{error::Error, io};
 use tracing_core::{span, subscriber::Interest, Event, Metadata};
 
 mod fmt_layer;
@@ -1196,7 +1199,7 @@ pub fn try_init() -> Result<(), Box<dyn Error + Send + Sync + 'static>> {
     #[cfg(not(feature = "env-filter"))]
     let subscriber = {
         use crate::{filter::Targets, layer::SubscriberExt};
-        use std::{env, str::FromStr};
+        use std::{env, eprintln, str::FromStr};
         let targets = match env::var("RUST_LOG") {
             Ok(var) => Targets::from_str(&var)
                 .map_err(|e| {
@@ -1255,6 +1258,7 @@ mod test {
             Subscriber,
         },
     };
+    use alloc::{borrow::ToOwned, string::String, vec::Vec};
     use std::{
         io,
         sync::{Arc, Mutex, MutexGuard, TryLockError},
diff --git a/tracing-subscriber/src/fmt/time/chrono_crate.rs b/tracing-subscriber/src/fmt/time/chrono_crate.rs
index def3b3b6..8b78bd28 100644
--- a/tracing-subscriber/src/fmt/time/chrono_crate.rs
+++ b/tracing-subscriber/src/fmt/time/chrono_crate.rs
@@ -1,7 +1,7 @@
 use crate::fmt::format::Writer;
 use crate::fmt::time::FormatTime;
 
-use std::sync::Arc;
+use alloc::{format, string::String, sync::Arc};
 
 /// Formats [local time]s and [UTC time]s with `FormatTime` implementations
 /// that use the [`chrono` crate].
@@ -108,8 +108,7 @@ impl FormatTime for ChronoUtc {
 /// the supported syntax.
 ///
 /// [`chrono::format::strftime`]: https://docs.rs/chrono/0.4.9/chrono/format/strftime/index.html
-#[derive(Debug, Clone, Eq, PartialEq)]
-#[derive(Default)]
+#[derive(Debug, Clone, Eq, PartialEq, Default)]
 enum ChronoFmtType {
     /// Format according to the RFC 3339 convention.
     #[default]
@@ -118,13 +117,12 @@ enum ChronoFmtType {
     Custom(String),
 }
 
-
 #[cfg(test)]
 mod tests {
     use crate::fmt::format::Writer;
     use crate::fmt::time::FormatTime;
 
-    use std::sync::Arc;
+    use alloc::{borrow::ToOwned, string::String, sync::Arc};
 
     use super::ChronoFmtType;
     use super::ChronoLocal;
diff --git a/tracing-subscriber/src/fmt/time/datetime.rs b/tracing-subscriber/src/fmt/time/datetime.rs
index d08ed558..48a6d055 100644
--- a/tracing-subscriber/src/fmt/time/datetime.rs
+++ b/tracing-subscriber/src/fmt/time/datetime.rs
@@ -192,7 +192,6 @@
 // permissive licensing, and of not having licensing issues being an
 // obstacle to adoption, that text has been removed.
 
-
 use std::fmt;
 
 /// A date/time type which exists primarily to convert `SystemTime` timestamps into an ISO 8601
@@ -333,7 +332,10 @@ impl From<std::time::SystemTime> for DateTime {
 #[cfg(test)]
 mod tests {
     use i32;
-    use std::time::{Duration, UNIX_EPOCH};
+    use std::{
+        format,
+        time::{Duration, UNIX_EPOCH},
+    };
 
     use super::*;
 
diff --git a/tracing-subscriber/src/fmt/writer.rs b/tracing-subscriber/src/fmt/writer.rs
index 52a20d69..e7c3e3b4 100644
--- a/tracing-subscriber/src/fmt/writer.rs
+++ b/tracing-subscriber/src/fmt/writer.rs
@@ -1,10 +1,13 @@
 //! Abstractions for creating [`io::Write`] instances.
 //!
 //! [`io::Write`]: std::io::Write
+
+use alloc::{boxed::Box, fmt, string::String, sync::Arc};
 use std::{
-    fmt,
+    eprint,
     io::{self, Write},
-    sync::{Arc, Mutex, MutexGuard},
+    print,
+    sync::{Mutex, MutexGuard},
 };
 use tracing_core::Metadata;
 
@@ -715,9 +718,7 @@ impl TestWriter {
 
     /// Returns a new `TestWriter` that writes to `stderr` instead of `stdout`.
     pub fn with_stderr() -> Self {
-        Self {
-            use_stderr: true,
-        }
+        Self { use_stderr: true }
     }
 }
 
@@ -1224,8 +1225,10 @@ mod test {
     use crate::fmt::format::Format;
     use crate::fmt::test::{MockMakeWriter, MockWriter};
     use crate::fmt::Subscriber;
+    use alloc::vec::Vec;
     use std::sync::atomic::{AtomicBool, Ordering};
     use std::sync::{Arc, Mutex};
+    use std::{dbg, format, println};
     use tracing::{debug, error, info, trace, warn, Level};
     use tracing_core::dispatcher::{self, Dispatch};
 
diff --git a/tracing-subscriber/src/layer/mod.rs b/tracing-subscriber/src/layer/mod.rs
index 12fab82c..0bf55efb 100644
--- a/tracing-subscriber/src/layer/mod.rs
+++ b/tracing-subscriber/src/layer/mod.rs
@@ -1675,7 +1675,6 @@ where
 
 feature! {
     #![any(feature = "std", feature = "alloc")]
-    #[cfg(not(feature = "std"))]
     use alloc::vec::Vec;
 
     macro_rules! layer_impl_body {
@@ -1773,8 +1772,6 @@ feature! {
         layer_impl_body! {}
     }
 
-
-
     impl<S, L> Layer<S> for Vec<L>
     where
         L: Layer<S>,
diff --git a/tracing-subscriber/src/layer/tests.rs b/tracing-subscriber/src/layer/tests.rs
index d7ad6176..02e16bd0 100644
--- a/tracing-subscriber/src/layer/tests.rs
+++ b/tracing-subscriber/src/layer/tests.rs
@@ -113,6 +113,8 @@ fn downcasts_to_layer() {
 
 #[cfg(all(feature = "registry", feature = "std"))]
 mod registry_tests {
+    use std::dbg;
+
     use super::*;
     use crate::registry::LookupSpan;
 
diff --git a/tracing-subscriber/src/lib.rs b/tracing-subscriber/src/lib.rs
index a3731448..619fd97b 100644
--- a/tracing-subscriber/src/lib.rs
+++ b/tracing-subscriber/src/lib.rs
@@ -161,6 +161,8 @@
 //! [`time` crate]: https://crates.io/crates/time
 //! [`liballoc`]: https://doc.rust-lang.org/alloc/index.html
 //! [`libstd`]: https://doc.rust-lang.org/std/index.html
+
+#![no_std]
 #![doc(
     html_logo_url = "https://raw.githubusercontent.com/tokio-rs/tracing/main/assets/logo-type.png",
     issue_tracker_base_url = "https://github.com/tokio-rs/tracing/issues/"
@@ -201,10 +203,11 @@
 // future, reducing diff noise. Allow this even though clippy considers it
 // "needless".
 #![allow(clippy::needless_update)]
-#![cfg_attr(not(feature = "std"), no_std)]
 
 #[cfg(feature = "alloc")]
 extern crate alloc;
+#[cfg(feature = "std")]
+extern crate std;
 
 #[macro_use]
 mod macros;
diff --git a/tracing-subscriber/src/registry/extensions.rs b/tracing-subscriber/src/registry/extensions.rs
index ff76fb59..afed2f7d 100644
--- a/tracing-subscriber/src/registry/extensions.rs
+++ b/tracing-subscriber/src/registry/extensions.rs
@@ -1,12 +1,12 @@
 // taken from https://github.com/hyperium/http/blob/master/src/extensions.rs.
 
 use crate::sync::{RwLockReadGuard, RwLockWriteGuard};
-use std::{
+use alloc::{boxed::Box, fmt};
+use core::{
     any::{Any, TypeId},
-    collections::HashMap,
-    fmt,
     hash::{BuildHasherDefault, Hasher},
 };
+use std::collections::HashMap;
 
 #[allow(warnings)]
 type AnyMap = HashMap<TypeId, Box<dyn Any + Send + Sync>, BuildHasherDefault<IdHasher>>;
diff --git a/tracing-subscriber/src/registry/mod.rs b/tracing-subscriber/src/registry/mod.rs
index 0fb03ad8..0acc3056 100644
--- a/tracing-subscriber/src/registry/mod.rs
+++ b/tracing-subscriber/src/registry/mod.rs
@@ -519,7 +519,10 @@ mod tests {
         prelude::*,
         registry::LookupSpan,
     };
-    use std::sync::{Arc, Mutex};
+    use std::{
+        sync::{Arc, Mutex},
+        vec::Vec,
+    };
     use tracing::{span, Subscriber};
 
     #[test]
diff --git a/tracing-subscriber/src/registry/sharded.rs b/tracing-subscriber/src/registry/sharded.rs
index 3004c9c5..64f0cdc5 100644
--- a/tracing-subscriber/src/registry/sharded.rs
+++ b/tracing-subscriber/src/registry/sharded.rs
@@ -10,10 +10,11 @@ use crate::{
     },
     sync::RwLock,
 };
-use std::{
+use core::{
     cell::{self, Cell, RefCell},
     sync::atomic::{fence, AtomicUsize, Ordering},
 };
+use std::thread_local;
 use tracing_core::{
     dispatcher::{self, Dispatch},
     span::{self, Current, Id},
@@ -536,7 +537,9 @@ mod tests {
     use crate::{layer::Context, registry::LookupSpan, Layer};
     use std::{
         collections::HashMap,
+        dbg, println,
         sync::{Arc, Mutex, Weak},
+        vec::Vec,
     };
     use tracing::{self, subscriber::with_default};
     use tracing_core::{
diff --git a/tracing-subscriber/src/registry/stack.rs b/tracing-subscriber/src/registry/stack.rs
index 4a3f7e59..2e682519 100644
--- a/tracing-subscriber/src/registry/stack.rs
+++ b/tracing-subscriber/src/registry/stack.rs
@@ -1,3 +1,4 @@
+use alloc::vec::Vec;
 pub(crate) use tracing_core::span::Id;
 
 #[derive(Debug)]
diff --git a/tracing-subscriber/src/util.rs b/tracing-subscriber/src/util.rs
index 1c98aa4d..6a48f255 100644
--- a/tracing-subscriber/src/util.rs
+++ b/tracing-subscriber/src/util.rs
@@ -1,5 +1,8 @@
 //! Extension traits and other utilities to make working with subscribers more
 //! ergonomic.
+
+#[cfg(feature = "std")]
+use alloc::boxed::Box;
 use core::fmt;
 #[cfg(feature = "std")]
 use std::error::Error;